In a manufacturing plant where semiconductor devices are manufactured, in a manufacturing process of, for example, semiconductor elements, ICs, liquid crystal panels, magnetic heads, or the like, static electricity may adversely affect electronic circuits and cause a change in electrical characteristics or damage to circuits. In addition, there is also a problem in that static electricity causes dust to be easily attached to products.
In particular, in the case of using an insulating substrate, electrostatic charging tends to occur, and the insulating substrate is formed of a material which is easily charged with static electricity, such as glass or a resin.
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (having a thickness of several nanometers to several hundred nanometers) formed over a glass substrate has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and thin film transistors that are used as switching elements in image display devices are particularly developed urgently.
In addition, in a manufacturing process of liquid crystal panels which are a typical example of image display devices, a reduction in thickness of glass by a reduction in weight, and an improvement in productivity by an increase in size of a glass substrate have been attempted.
There are various kinds of metal oxides, which are used for a wide range of applications. Indium oxide is a well-known material and is used as a light transmitting electrode material which is needed for liquid crystal displays and the like. Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors in each of which such a metal oxide having semiconductor characteristics is used for a channel formation region have already been known (Patent Documents 1 and 2).